It's a cold, wet evening in early January, and Fi Spotswood is telling me about the "sleep monsters".

"I've seen faces staring out of the rocks," she shouts over the noise of rushing water. "Grumpy faces, sad faces, gremlins, trolls, ogres, fairies – all sorts of magical beings."

Perversely for the time of year, the time of day (just getting dark), the weather conditions (cold and starting to rain), and the state of the river (in danger of bursting its banks), Fi and I are kayaking down the River Wye near the village of Tintern in Monmouthshire.

The last time I was here I had come, like most sensible visitors, on a beautiful summer's day to wander around the ruins of the 12th-century Tintern Abbey, and to ponder over Wordsworth's poem of the same name. But it's a different story this time.

Fi's sleep monsters – hallucinations conjured by a mixture of exhaustion and sleep deprivation – haven't shown up yet, but at this moment they seem a much more likely proposition than Wordsworth's more sublime imaginings.

This is my first experience of an "adventure race", a team sport that combines outdoor activities ranging from running and trekking to kayaking, mountain biking and climbing. Races can last from two hours to several days, and attract a broad cross-section of competitors, from enthusiastic amateurs to full-blown, sponsor-bedecked Olympic-class athletes.

Not content with easing myself in gently, I have leapt in at the deep end and joined the British team in training for an "expedition class" race. These are events at the extreme end of the spectrum – lasting from three to 11 days and held in some of the world's most challenging wildernesses.

The Wenger Patagonian Expedition Race, which starts the week after next in Chile, is billed as "the wildest, most remote race in the world".

Designed to be a true expedition, the race is run by 15 mixed-sex teams of four who must navigate with just a compass, altimeter and rudimentary maps across more than 400 miles of the remotest parts of Chilean Patagonia in under nine days. Disciplines include running, mountain biking, kayaking and climbing; en route crossing steppes, plains, mountains, glaciers, forests, rivers, swamps, lakes and mushy peat bogs.

The Wenger Patagonian is the brainchild of a Chilean geologist, Stjepan Pavicic, whose love of adventure is accompanied by a desire to spread the word about the urgent need to protect his native Patagonia from logging companies and other unsustainable industries.

The race was first run in 2004 and now attracts teams and reporters from all over the world. The course changes every year and is not revealed until the day before the start. Each team must decide on the amount and type of food it will carry to last the full nine days. In addition to some mandatory items of equipment, it must also decide how much and what equipment it will carry.

Barring the kayaking sections, there are no official "dark periods" when the clock stops ticking, so the teams must choose if, and when, to rest and sleep.

This means that, as well as physical and mental endurance, the race demands a high level of organisation and strategy. Both the 2009 and 2010 races were won by the British team, the former in just six-and-a-half days and in such savage weather that only two other teams managed to finish.

Two members of that team, Bruce Duncan and Mark Humphrey, are competing again this year as Team adidas TERREX, alongside Fi Spotswood and Nick Gracie.

Back in Wales and with less than a month to go before the start of the race, the team is putting me through my paces. I am beginning to realise that I will be lucky to survive what for them is a mere stretch of the legs, but which for me is the gruelling prospect of six hours of running, kayaking, mountain biking and abseiling over the hills and valleys around Tintern.

Although I am "fit for my age" (a convenient catch-all with an inbuilt get-out clause), I am not sure I am remotely fit enough to survive the 30-mile day/night route we have planned. Embarrassingly, I find myself winded at the first hint of a slope. But my first insight into the teamwork of adventure racing proves to be my salvation.

Nick drops back and starts telling me an anecdote from a previous Patagonian race when the team found themselves bogged down in the "Forest of Doom", a Mordor-like landscape of dense moss-covered ancient trees where the ground kept collapsing beneath them.

Mind over matter is an interesting phenomenon, and by being distracted I am soon climbing the steep walls of the valley once more without too much distress. And later, when a particularly steep section kicks in, the team come to the rescue once again.

"Time for the bungee cords," Nick smiles, attaching me to his lightweight backpack. At first I am reluctant, as I feel I am dragging the team back, but Nick soon puts me right. "Quite the opposite," he tells me. "We can only travel as fast as the weakest link. And every one of us will be the weakest link at some stage. If you're feeling strong the added drag doesn't seem to make much difference, but it can really help if you're the one flagging."

Nick's intervention soon does the trick, and as we reach the top of the ridge my spirits are revived by views across the valleys worthy of the bard himself, with the low winter sunlight slanting across the uplands beyond. My lungs are feeling freer and energy is once again coursing through my veins. Perhaps, just perhaps, I can survive this after all.

On the downhill stretch, I am soon dancing down the muddy, root-covered path, desperately trying to avoid tripping, slipping or falling at every turn. Later, after nightfall and with lights on our helmets, the problem of balance is even more critical. What on earth this would be like on Day Five in sleep-deprived Chilean Patagonia, I can only imagine.

As we go, I hear a lot about the importance of speedy "transitions", and after our kayak down the swollen Wye, the logistics are complicated still further by the fact that it is now dark and we are all wet and cold. But refuelling and concentrating on the next stage is all-important. Races can be won and lost in the complicated logistics of changing kit, adjusting lights and taking on food and fluids. Mistakes – leaving an important piece of kit behind, for example – can have severe consequences, especially when you are battling fatigue.

The mountain biking section, which I am hoping will play to my strengths, in fact nearly defeats me. As we start the climb up a narrow one-in-seven hill, I just can't get my momentum right or the bike in the correct gear. But I soldier on once again, and the team's bonhomie is infectious. Once again I experience the roller-coaster ride of going from burning lungs and sagging spirits one minute to a surge of adrenalin the next as I am urged on by the bedraggled but smiling faces of the others. The sense of achievement is already palpable, and the addictive quality of adventure racing is dawning on me.

Our day finishes with another uphill burst, a short abseil down a forest rock-face, and a taster of what attempts to sleep will be like when the team reaches Patagonia. The lightweight tent with no sleeping mats to cushion us from the cold, hard earth provides no more than rudimentary protection. A sardine tin would feel more spacious. But this is all part of the strategy too, as retaining body heat is critical in any expedition race.

"Choosing when and where to sleep is vitally important and can make the difference between winning and losing the race," the team's captain and chief navigator, Bruce Duncan, tells me later over a well-earned pint. "It is sometimes far better to sleep and let a team overtake you in the short term than battle on only to find you can't find anywhere to shelter later on."

My day in the Welsh hills with this inspirational team was nothing compared to what they will soon have to endure in Patagonia. But even this short taster opened my eyes as to why the competitors are prepared to endure so much for the privilege of competing in this extraordinary race.

"It's the landscapes we travel through," says Fi. "These are places where people have rarely, if ever, been before and they are some of the most beautiful places on Earth. It's that and the fact that you experience all the highs and lows with friends who are prepared to go through all these experiences with you. It's pure magic."

And not even an army of sleep monsters will change that.

The Wenger Patagonian Expedition Race (www.patagonianexpeditionrace.com) runs every year in Chilean Patagonia. The next race will take place from February 8 to16.

The progress of the adidas TERREX Adventure Racing Team and the other 14 teams can be followed on the official race website.

This year it is hoped the race will raise US$20,000 to protect the Patagonian huemul, a critically endangered species of deer that exists in only a few small pockets of Chile.

Richard Madden and members of the team will be giving a talk about the race at 12pm on January 29 2011 in The Daily Telegraph Adventure Forum at Adventure Travel Live (www.adventuretravellive.com).

Adventure racing is a rapidly growing sport and welcomes people of all ages and all standards. The Sleep Monsters website (www.sleepmonsters.co.uk) has details of races ranging from short tasters to multi-day events for the more experienced.

Open Adventure (www.openadventure.com) runs a series of races suitable for beginners and experienced participants alike, including set-time events of five, 12 and 24 hours in which competitors have to visit as many checkpoints as possible in the time available.